Very interesting, a permission problem could be causing you all your problems. Try to remove all yafaray and the bounty folders from the Blender addons folder. If you cannot because of permission problems, try to do it as a Windows administrator.

Once physically removed, close and open Blender and go to user preferences, addons. At the bottom you wll see addons in yellow that are enabled but the files are missing. Disable them now there.

Unfortunately I have been extremely busy at work over the last months so I could not dedicate time to YafaRay lately.

About materials, from what Ive investigated so far it is very difficult for me to understand how the materials/integrators math works in YafaRay, much less to fix it. So Im becoming more and more convinced I might have to rewrite most of the rendering/integration codev

Now Im waiting to see whether we can/should change the YafaRay license to LGPLv3 so we can use Disney BRDF code and perhaps replace the rendering core code by Intel Embree (only a maybe!)

Once we have an answer about the license and I have a clearer view of the path forward I expect to become more active again...